1. Field of the Invention
The present invention relates to a lens barrel of a type capable of fixedly holding a plastic lens in a location where the plastic lens is placed in a desired axial position and an optical axis of the plastic lens is substantially coaxial with a geometric axis of the lens barrel.
2. Description of Related Art
There has been known a projector equipped with a projection lens system for enlarging and projecting an image modulated by an image display element on a remote screen The projection lens system is capable of changing its magnification power by shifting an axial position of a front lens (a distance between a front lens and a neighboring lens). Since it is common to use a super-wide-angle lens for the front lens, a plastic lens, that can be comparatively easily manufactured at low costs by molding even though it is complicated in shape, is often used for the front lens.
Conventionally, in incorporating the projection lens system in a lens barrel, the front lens is adjusted in an axial position by inserting a spacer having an appropriate thickness, which is selected from spacers different in thickness, between a back face of the front lens and a bearing wall of the lens barrel. However, it is troublesome and expensive to always have a stock of various thickness of spacers ready.
In order to skirt around this advantage, it has been proposed in Unexamined Japanese Patent Publication Nos. 2002-350702, 2004-212822 and 2005-49599 to adjust an axial position of a lens element by selectively using a plurality of shoulders different in axial position which are formed in either one of the lens element or a lens holder for the lens element and a bearing wall of a lens barrel. Further, it has been proposed in Unexamined Japanese Patent Publication No. 10-319291 to incorporate an adjuster ring having locating means between a back face of a lens element and a bering wall of a lens barrel. The locating means comprises a plurality of axial projections extending from the back of the lens element and a multiple sets of locating recesses different in axial depth by set so that when placing the lens element in a desired axial position in the lens barrel, the adjuster ring is turned so as to receive the axial projections by an appropriate set of locating recesses having an appropriate axial depth.
Meanwhile, injection molding is commonly used to manufacture plastic lenses. However, the problem encountered by such a plastic lens is that a molded plastic lens possibly has eccentricity, i.e. a deviation of a geometric axis decentered from an optical axis of the plastic lens, due to dimensional differences of the molded plastic lens from a design specification which occur depending upon marginal production accuracy of a molding die. In particular, when using a multi-molding die having a plurality of molding cavities, plastic lenses molded by the multi-molding die are apt to have eccentricity different from one another according to the molding cavities.
When incorporating such a plastic lens having eccentricity into a lens barrel, the problem encountered by the plastic lens is that it is hard to bring an optical axis of the plastic lens almost coaxial with a geometric axis of the lens barrel in the case where the technique of axial lens position adjustment described in Unexamined Japanese Patent Publication No. 2004-212822 or No. 10-319291, or that, it is possible but somewhat troublesome to bring an optical axis of the plastic lens almost coaxial with a geometric axis of the lens barrel in despite of the capability of axial adjustment in the case where the technique of axial position adjustment of a lens described in Unexamined Japanese Patent Publication No. 2002-350702 or No. 2005-49599 2004-212822. In addition, since it is essential to fixedly hold the plastic lens in the lens barrel with an adhesive, it is hard to remove the plastic lens from the lens barrel after having been incorporated in the lens barrel once. If using a projection lens system including a lens element whose optical axis remains decentered from a geometric axis of a lens barrel, the projection lens system entails quality deterioration of an image projected thereby.